Form keys and method for thermoforming undercuts in foam parts

ABSTRACT

A pair of form keys and a method for forming reverse drafts in foam parts are disclosed. A male form key is reciprocally mounted in a first mold half in a retracted position. A female form key is rotatably mounted in a second mold half in a neutral position. The first mold half includes a first drive pin that when engaged, moves the male form key into engagement with a foam sheet to form an undercut as the first and second mold halves are closed. The first mold half also includes a second drive pin that engages the female form key as the first and second mold halves close rotating the female form key into engagement with the foam sheet and the male form key. After closure of the mold and the product is formed, the mold is opened. As this occurs, the drive pins are disengaged and the male form key retracts and the female form key rotates to their neutral positions. The formed product may then be stripped from the mold while maintaining the reverse draft or undercut.

FIELD OF THE INVENTION

[0001] The present invention relates generally to form keys and a methodfor using form keys for thermoforming undercuts in matched molded foamparts. More particularly, the invention relates to form keys for forminga reverse draft in thermoformed parts, and to a method for thermoformingundercuts in parts using matched metal molds and for stripping the partfrom the molds while maintaining the undercuts.

BACKGROUND OF THE INVENTION

[0002] Foam containers for restaurants, schools, hospitals and the likeare typically formed by placing a foam sheet between a pair of molds andclosing the molds to thermoform a part. If the part requires, forexample, a lock for a hinged lid container, a reverse draft or undercutcan be formed, but forming a reverse draft or undercut for a lock in thepart, particularly foam parts, has been a problem. A reverse draft hasbeen difficult to form in foam containers such as polystyrene foamcontainers due to an inability to strip the formed part from the toolingwhile maintaining the desired shape of the reverse draft. For thisreason, a continuous thermoforming process for forming parts, with areverse draft has not been possible. If this problem could be overcome,this would allow for different closure designs in parts, and thedevelopment of continuous thermoforming processes for forming foamparts.

SUMMARY OF THE INVENTION

[0003] The present invention is directed to form keys for forming areverse draft in a thermoformed part, for example, a polystyrene foampart such as a hinged lid container, and to a method for thermoforming areverse draft or undercut in the part and stripping the part from a moldwhile maintaining the shape of a reverse draft or undercut. The form keyis part of a mold used to thermoform foam parts. The mold includes twohalves, a first half with a forming cavity and a second half with aforming plug. A male form key is reciprocally mounted in the first moldhalf and reciprocates into and out of the forming cavity in response toengagement with a first drive pin which is reciprocally mounted in thefirst mold half and moved by engagement with the second mold half. Afemale form key is rotatably mounted in the second mold half and isrotated between a first and a second position upon engagement with asecond drive pin mounted on the first mold half.

[0004] The method of thermoforming foam parts includes placing a foamsheet between a forming cavity mold and a forming plug mold eachincluding the male form key or the female form key as described above.Prior to thermoforming, the male form key is biased to a retractedposition out of a cavity and the female form key is biased to anon-engaging position. The forming cavity mold and forming plug mold arethen brought together. As this occurs, the male form key and female moldkey are moved into the foam sheet forming an undercut or reverse draftin a part formed from the foam sheet. The forming cavity mold andforming plug mold are then moved apart. As this occurs, the male formkey and female form key are moved away from the formed part and theformed part can be stripped from the forming cavity and plug molds whilemaintaining the undercut.

[0005] Although the present invention is disclosed for matched metalmolds and thermoforming foam parts, the invention is applicable tonon-matched metal molds and forming parts from solid sheets.

BRIEF DESCRIPTION OF THE DRAWINGS

[0006] Other objects and advantages of the invention will becomeapparent upon reading the following detailed descriptions and uponreference to the drawings in which:

[0007]FIG. 1 is an enlarged cross sectional view of a thermoformed partincluding a reverse draft or undercut formed in accordance with theprinciples of the present invention;

[0008]FIG. 2 is a cross sectional view of a mold in an open position andused to mold the formed part illustrated in FIG. 1;

[0009]FIG. 3 is an enlarged cross sectional view of a portion of themold in the closed position showing the male and female form keys inpositions engaging the foam part and forming a reverse draft orundercut; and

[0010]FIG. 4 is a view similar to FIG. 3 with the mold in the openposition after the foam part has been thermoformed with a reverse draftor undercut.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

[0011] Referring now to the drawings, a thermoformed part 10 with areverse draft or undercut 12 formed in accordance with the principles ofthe present invention is illustrated in FIG. 1. The part 10 is of a foammaterial such as polystyrene foam formed using matched metal tooling. Itshould be recognized, however, that the principles of the presentinvention are also applicable to sheet material and non-matched metaltooling. In the embodiment illustrated, the part 10 is a lid or coverfor a foam container that can be used by restaurants, schools,hospitals, hotels, food distributors, and the like for containing itemssuch as food. The reverse draft or undercut 12 of this embodiment servesas part of a locking structure to lock the part or cover 10 to anotherpart, such as a tray, of a container. The ability to form the reversedraft or undercut 12 in foam products allows for different closure orlocking designs and leak resistant foam containers.

[0012] In the past, thermoforming a reverse draft or undercut withmatched metal tooling has had the problem of an inability to strip aformed part, such as part 10, from the matched tooling while maintainingthe shape of the reverse draft or undercut 12. The matched tooling ormold 14 shown in FIGS. 2-4 overcomes this problem. The mold 14 includesa first mold half 16 defined by a forming cavity 18 that is attached toa top platen 20. The forming cavity 18 defines a cavity 19.

[0013] The mold 14 also includes a second mold half 22 defined by aforming plug 24 attached to a bottom platen 26. The forming plug 24defines a plug 27 of a complimentary configuration to the cavity 19. Theundercut 12 is formed by male form keys 28 reciprocally mounted in theforming cavity 18 and female form keys 30 rotatably mounted in theforming plug 24. The male form keys 28 are located in a slot 32 in theforming cavity 18 and are biased by biasing elements such as springs 34to a retracted position out of the cavity 19. A force to move the formkeys 28 to a position extending into the cavity 19 is provided by firstdrive pins 36 each reciprocally mounted in the forming cavity 18 andbiased to a first position (shown in FIGS. 2 and 4) by a biasing elementsuch as a spring 38. The first drive pins 36 include a cut out 40 and aninclined ramp 42. In the neutral position of the first drive pins 36(FIGS. 2 and 4), the male form keys 28 are in the slot 40 above the ramp42. In this neutral position the male form keys 28 are retracted out ofthe cavity 19 under the biasing force of the springs 34. The formingcavity 18 further includes second drive pins 44 that interact with thefemale form keys 30.

[0014] The female form keys 30 each includes a stem 46 to which abiasing member such as a spring 48 is attached. The springs 48 bias eachfemale form key 30 to a first, neutral position (FIGS. 2 and 4). Eachfemale form key 30 also includes a pin engagement surface 50 and anundercut surface 52.

[0015] To thermoform a foam part 10, a sheet of foam material 54 ispositioned between the first mold half 16 and the second mold half 22while the mold 14 is in the open position (FIG. 2). The thermoformingprocess is commenced by bringing the first mold half 16 and the secondmold half 22 together (FIG. 3). As this occurs, the first drive pins 36engage the second mold half 22 moving the first drive pins 36 upwardlycausing the inclined ramp 42 to engage the male form keys 28. Thisengagement moves the male form keys 28 into the forming cavities 18 andinto the sheet of foam material 54. At the same time, the second drivepins 44 engage the sheet of foam material 54 and the pin engagementsurfaces 50 on the female form keys 30. This engagement rotates thefemale form keys 30 into engagement with the sheet of foam material 54and the male form key 28 forming the reverse draft or undercut 12 uponclosure of the mold 14.

[0016] Once the foam part 10 and the reverse draft or undercut 12 havebeen formed, the mold 14 is opened (FIG. 4). As this occurs, the drivepins 36 and 44 are moved out of engagement with the second mold half 22and return to their neutral positions (FIG. 4). The male form keys 28and the female keys 30 also return to their neutral positions out ofengagement with the formed part 10 allowing sufficient clearance for theformed part 10 to be stripped from the mold 14 without interferencewhile maintaining the shape of the reverse draft or undercut 12.

[0017] What has been described are mechanically driven form keys 28 and30 that allow for undercuts 12 to be thermoformed into foam parts 10using matched metal molds 14. This structure allows the foam part 10 tobe stripped from the mold 14 without damage to the undercut 12 or havingthe part 10 hang up in the mold 14 thus allowing for a continuousthermoforming process. This allows the design of different closuredesigns in foam parts. As previously noted, the present invention mayalso be used in a thermoforming process using non-matched metal molds.

[0018] While the present invention has been described with reference tothe particular embodiments illustrated, those skilled in the art willrecognize that many changes and variations may be made thereto withoutdeparting from the spirit and scope of the present invention. Theembodiments and obvious variations thereof are contemplated as fallingwithin the spirit and scope of the claimed invention, which is set forthin the following claims:

What is claimed is:
 1. A mold with form keys for forming a reverse draftin a thermoformed foam part, comprising: a first mold half including aforming cavity, a male form key reciprocally mounted in said first moldhalf for movement between a first position extending into said cavityand a second position retracted out of said cavity, a first biasingmember engaging said male form key and biasing said male form key tosaid second position, a first drive pin mounted in said first mold halfand a second drive pin mounted in said first mold half, said first drivepin moveable from a first position to move said male form key to saidfirst position, said first drive pin movable to a second positionallowing said male form key to move to its second position, and a secondmold half including a forming plug, a female form key rotatably mountedin said second mold half and rotatable between a first position and asecond position, said second mold half including a portion for engagingsaid first drive pin, said female form key including a portion engagedby said second drive pin whereby said engagement of said second drivepin rotates said female form key from said first position to said secondposition of said female form key.
 2. The mold claimed in claim 1comprising a second biasing member engaging said female form key andbiasing said female form key to said first position.
 3. The mold claimedin claim 1 said male form key including a portion in a configurationcorresponding to the configuration of said reverse draft to be formed,and said female form key including a portion complementary to saidportion of said male form key.
 4. A method of thermoforming undercuts infoam parts using matched metal molds and stripping the foam part fromthe molds while maintaining the shape of the undercut, comprising:placing a foam sheet between a forming cavity having a retractable maleform key and a forming plug having a rotatable female form key, biasingsaid male form key to a position retracted out of said forming cavity,biasing said female form key to a non-engaging position, moving saidforming cavity and said forming plug together, moving said male form keyinto said foam sheet, moving said female form key into said foam sheetand against said male form key to form an undercut, and closing saidforming cavity forming a foam part.
 5. The method claimed in claim 4comprising separating said forming cavity and said forming plug,retracting said male form key, moving said female form key from saidfoam sheet, and stripping said foam part from said forming cavity andsaid forming plug.
 6. The method claimed in claim 4 wherein moving saidmale form key comprises engaging a first drive pin on said formingcavity with a portion of said forming plug and moving said first drivepin against said male form key.
 7. The method claimed in claim 4 whereinmoving said female form key comprises engaging said female form key witha second drive pin mounted on said forming cavity.
 8. Mechanicallydriven form keys for forming undercuts in foam parts thermoformed usingmatched metal molds, comprising: a first mold half including a formingcavity, at least one male form key mounted on said first mold half formovement between a first position wherein said male form key isretracted out of said cavity and a second position wherein said maleform key extends into said cavity, a first drive pin reciprocallymounted in said first mold half for movement between a first positionout of engagement with said male form key and a second position inengagement with said male form key, and a second mold half including aforming plug, at least one female form key rotatably mounted in saidsecond mold half rotatable between an undercut forming position and aproduct release position.
 9. The mechanically driven form keys claimedin claim 8, comprising a second drive pin mounted on said first moldhalf in a position to engage and rotate said female form key to saidundercut forming position as said first mold half and said second moldhalf are moved together.
 10. The mechanically driven form keys claimedin claim 8, comprising a first biasing member in said first mold halfengaging said male form key and biasing said male form key to beretracted out of said cavity, a second biasing member in said first moldhalf engaging said first drive pin and biasing said first drive pin tosaid first position, and a third biasing member in said second mold halfengaging said female form key and biasing said female form key to saidproduct release position.